Joint ramming cement for electrolytic reduction cell cathodes

ABSTRACT

A ramming cement suitable for joining a plurality of adjacent carbon blocks having joint spaces between them to form a continuous cell lining structure for an electrolytic cell, such as an aluminum reduction cell, is prepared from a mixture of a petroleum pitch having a cube-in-water softening point between about 40* and about 85*C, and containing not more than about 1% of quinoline-insoluble material, and an aggregate such as calcined anthracite.

United States Patent 1191 Reamey et al.

JOINT RAMMl NG CEMENT FOR ELECTROLYTIC REDUCTION CELL CATHODESInventors: Herbert K. Reamey; Jerry F.

Newman, both of Hot Springs, Ark.

Reynolds Metals Company, Richmond, Va.

Filed: Oct. 31, 1973 Appl. No.: 411,537

Assignee:

US. Cl 204/243 R, 204/294, 252/502, 252/510 Int. Cl. C22d 3/02, BOlk3/08, l-l01b 1/04 Field of Search 204/294, 243 R, 244-247; 252/502, 510,511

References Cited UNITED STATES PATENTS l/l963 Myers et al. 204/294 X[451 Mar. 18, 1975 3,187,089 6/1965 Cosby et al. 204/294 X 3,576,7004/l97l 3,666,653 5/1972 3,764,509 10/1973 Etzel et a] 204/294 X PrimaryE.\'aminer.lohn H. Mack Assistant ExaminerD. R. Valentine Attorney,Agent, or Firm-Glenn, Palmer, Lyne & Gibbs [57] ABSTRACT 5 Claims, NODrawings BACKGROUND OF THE INVENTION This invention relates toelectrolytic reduction cells provided with a prebaked carbon blocklining which functions as the cathode, and to an improved carbonaceousramming cement for binding the carbon blocks by filling the jointsbetween them. More particularly, the invention concerns an improvedramming cement for aluminum reduction cell cathodes.

Aluminum metal is conventionally produced in electrolytic cells bypassing a current through a bath of molten cryolite containing dissolvedalumina, in a large tank or cell lined with carbon, the lining servingas part of the cathode system. Large carbon blocks presented at the topof the bath function as the anode. Molten aluminum metal collects as ametal pad at the bottom of the cell and is removed periodically. Thecell may have a bottom lining made up of cemented carbon blocks in whichhorizontally disposed steel cathode bars extend from the opposite sidesof the cell. These cathode bars are connected to the cathode collectorblocks by means of cast iron, and the blocks are anchored in a sidelining of rammed carbon.

Most of the aluminum reduction cells in commercial use employprefabricated carbon blocks as the cell lining and as the cathodicworking surface. These blocks provide high operating strength, higherdensity, lower porosity, and lower resistance than that of a continuousrammed paste type of lining. These blocks are formed into a liquid-tightcontainer surface by filling the joints between the blocks with a rammedcarbonaceous cement. This liquid tightness is an important factor indetermining the life of a reduction cell, which depends to aconsiderable degree on the extent and rate of electrolyte penetrationinto the cell bottom.

The best types of cathode blocks currently in use are considered to bethose manufactured from calcined anthracite or calcined mixtures ofanthracite with other forms of carbon such as metallurgical coke orgraphite.

The prior art ramming cements have usually been prepared by mixing agraded calcined anthracite aggregate with a binder. The binder has beenconventionally a coal tar type pitch. These materials are heatedtogether to a consistency such that the mixture can be rammed as a hotmelt into the joints between the cathode blocks. Thus, for example, itis know to prepare a joint material for aluminum reduction cell cathodeblocks from calcined anthracite fines and a carbonizable binder such asa coke-over pitch having a softening temperature of about 75C.

A characteristic of ramming cements made with coal tar or coke ovenpitches has been a tendency for the binder to migrate and be absorbedinto the pores of the cathode blocks. This has been considereddetrimental in the art, and steps have been taken to offset it, such asprecoating the blocks with a separate pitch coating. Coal tar or cokeoven pitches are also characterized by a considerable content ofmaterial insoluble in quinoline, ranging from about to by weight, whichmay affect the migration properties of the pitch binder. Anotherapproach taken in the prior art has been to employ relatively highsoftening point coal tar pitches, e.g. 130 to 160C, in admixture with apolymerizable monomeric plasticizer to improve the properties of thebinder mixture.

GENERAL DESCRIPTION OF THE INVENTION In accordance with the presentinvention there is provided a novel ramming cement for electrolytic cellcathodes which is readily prepared from commercially availablematerials, and which will ram firmly with good interparticle contactinto the joint spaces between the cathode blocks, and which will notshrink when the lining assembly is subsequently baked.

It has been found, surprisingly and unexpectedly, that a certain type ofbinder pitch possesses the quality of being absorbed by the rammingcement aggregate, such as calcined anthracite, to such an extent thatthe resulting mixture can contain an excess quantity of pitch binderwhich will then bleed during subsequent baking after application to thecathode block joints, so that the joint cement and block assembly willform an impenetrable and inseparable mass. It has been discovered thatin this relationship, the bleeding tendency of the pitch is not only notdetrimental, but in fact contributes to the production of a superiorramming cement.

The essential feature of the improved ramming cement of the inventionlies in the use ofa type of binder pitch that is very low in its contentof quinolineinsoluble material, preferably less than about l7c byweight. The quinoline-soluble material present in considerably greaterquantity in the prior art binder pitches is now believed to block thepore structure of the aggregate and to prevent absorption of the binderinto the pores of the aggregate and the cathode blocks.

A ramming paste prepared with a pitch which does not readily absorb intothe pores must carry any excess pitch in the voids between theparticles,which prevents ramming firmly with good particle contact and leads to aspongy structure. Upon baking, this type of ramming paste shrinks,causing a poor joint at the interface between the cement and the carbonblock, and acting as a source of weakness and potential failure in thecathode structure.

For the purposes of the present invention, a suitable type of binderpitch is a petroleum pitch having a softening point (cube test) betweenabout 40 and about C. (cube-in-water melting point), preferably about 55to 60 C., and a content of material insoluble in quinoline not greaterthan about 1% by weight. Petroleum pitch having these characteristicsmay be prepared from residual aromatic tar from the high temperaturecracking of gas oil by steam cracking followed by vacuum distillation ofvolatile fractions. The resulting pitches have been proposed as bindersin the body of prebaked or Soderberg type carbon electrodes, not, so faras known, as a binder between preformed carbon blocks as a rammingcement ingredient, which presents a different problem.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the presentlypreferred practice of the invention, as illustrated with reference to analuminum reduction cell, the bottom and walls of the cell hull are linedwith preformed carbon wall blocks of conventional type to form a cathodecarbon lining. These carbon blocks, in place, are joined or bonded byramming into the joint spaces between the walls of adjacent blocks, theimproved ramming cement of the invention, to form a continuous rammedmonolithic lining. The assembled lining is then baked in situ at asufficient temperature and for a sufficient period of time to carbonizethe binder ingredients while at the same time avoiding undue shrinkage.

The ramming cement of the present invention is composed of a mixture ofa suitable aggregate and the petroleum pitch having the characteristicspreviously described, forming a mastic. The aggregate is preferablycalcined anthracite, although a mixture of calcined anthracite andcalcined metallurgical coke or other car bonaceous material such asgraphite, may also be employed. Advantageously the aggregate is 100%calcined anthracite, about 30% of which has a nominal particle diameterof about three-eighths inch, the balance representing materialcomminuted to a fineness such that nominally about 50% will pass througha 200 mesh Tyler screen. This degree of fineness may be achieved bygrinding in a ball mill or other suitable device.

In the preparation of the ramming cement, a preferred procedure,according to the invention, is to melt the pitch in a separate vessel,bringing it to a temperature of about l30C., or within a range of 120 to140C. This melted pitch is then added to the aggregate in a mixerprovided with heating means, in which the mixture is maintained at atemperature between about 150 to 190C. for approximately one hour. Atthe end of this time, the ramming paste will have attained a consistencysuch that a coherent ball can be formed in gloved handles, which will befirm and not crumble. If too much binder is used, the paste will bespongy and will not compact firmly upon compacting with a pneumatichammer. The ramming paste when properly prepared, has a consistencysimilar to that of hot asphalt mastic used in paving of roads.

The proportion of binder pitch in the ramming paste is between about 13%and about 16.5% by weight, preferably between about 13% and about 13.5%.

The petroleum pitch employed, having a cube-inwater melting temperaturebetween about 40 and about 85C., typically about 55C., is preferably onehaving a Conradson carbon value above 45%.

In forming a cathode lining, the joints are formed by transferring theramming cement into the approximately two-inch joint spaces between theprebaked cathode blocks, and compacting it with pneumatic hammers. Thejoint is formed in several layers to a smooth level surface flush withthe top of the cathode block. The sidewalls are formed in a similarmanner by ramming the cement between suitably shaped forms to providethe cathodic cavity of the reduction cell.

The cathode blocks themselves are made from graded calcined anthracite,or from mixtures of calcined anthracite, metallurgical coke, lamp black,or graphite. A typical formulation may comprise calcined anthracite,with between about 12% and about graphite flour by weight. The blockingredients are admixed with a suitable binder pitch in a heated mixerand formed into shape by extrusion or molding, according to conventionalpractice. A typical cathode block may have dimensions of 14 inch lengthby 14 inches width, and 90 inches overlength, with a suitable slot, forthe cathode collector bar which is either molded or machined into thecathode block. These blocks are prebaked at temperatures of about 1200Cby packing into carbon baking furnaces. Generally, the cathode blocksalso have small grooves molded or machined into the side surfaces wherethe joints will be formed to assist in keying in the joint cement. Theslots are relatively small, about three-eighths inch deep.

After the lining and joints have been properly formed and the celldeckplate and other parts are in place, any suitable method may beemployed to place the cell in service and to bake the cathode and jointassembly. A method commonly used is to insert a layer of metallurgicalcoke of particle size approximately one-half inch diameter in the spacebetween the anode and cathode, and then employ this carbon bed as aresistant heating means for the passage of current between the anode andthe cathode. When the elecrodes are not, cryolite and/or molten aluminumare added to start the cell in operation. The process heat of the cellwill then serve to complete the carbonization of the joints andsidewalls.

THE DESCRIPTION OF THE PREFERRED EMBODIMENTS The following example willserve to illustrate the practice of the invention, but is not to beregarded as limiting.

EXAMPLE 1 Preparation of Ramming Cement Approximately 15 pounds of apetroleum pitch having a Conradson value of 50%, and aquinolineinsoluble material content of 0.80% were melted at atemperature of 55C and heated further to about C. The melted pitch wasadded to 100 pounds of calcined anthracite aggregate, of which about 30%had a nominal three-eighths inch average particle size, and the mixture(containing about 13.25% pitch) was stirred to a uniform consistency forabout one hour at a temperature of C. The resulting ramming paste wasthen applied to a cathode block lining of a1 aluminum reduction cell asdescribed above.

What is claimed is:

1. Process for forming the cathode lining of an electrolytic cell, saidlining being made up of adjacent carbonaceous blocks having joint spacestherebetween, comprising the steps of introducing into said joint spacesa ramming cement comprising a mixture of pitch and a carbonaceousaggregate, at a temperature sufficient to maintain suitable consistencyfor compacting the cement in said spaces, and then baking the blocks andcemented joints at a temperature and for a period of time sufficient tocarbonize the cement and to form a continuous lining structure, saidmixture containing from about 13 to about 16.5% by weight of petroleumpitch characterized by having a cube-in-water softening point betweenabout 40 and about 85C. and having a content of material insoluble inquinoline not greater than about 1%.

2. The process of claim 1 in which said electrolytic cell is an aluminumreduction cell.

3. The process of claim 1 in which said aggregate is calcinedanthracite.

4. The process of claim 1 in which said aggregate is a mixture ofcalcined anthracite and calcined metallurgical coke.

5. The process of claim 1 in which said aggregate is predominantlyfinely divided material of which about 50% passes a 200 mesh screen.

1. PROCESS FOR FORMING THE CATHODE LINING OF AN ELECTROLYTIC CELL, SAIDLINING BEING MADE UP OF ADJACENT CARBONACEOUS BLOCKS HAVING JOINT SPACESTHEREBETWEEN, COMPRISING THE STEPS OF INTRODUCING INTO SAID JOINT SPACESA RAMMING CEMENT COMPRISING A MIXTURE OF PITCH AND A CARBONACEOUSAGGREGATE, AT A TEMPERATURE SUFFICIENT TO MAINTAIN SUITABLE CONSISTENCYFOR COMPACTING THE CEMENT IN SAID SPACES, AND THEN BAKING THE BLOCKS ANDCEMENTED JOINTS AT A TEMPERATURE AND FOR A PERIOD OF TIME SUFFICIENT TOCARBONIZE THE CEMENT AND TO FORM A CONTINUOUS LINING STRUCTURE, SAIDMIXTURE CONTAINING FROM ABOUT 13 TO ABOUT 16.5% BY WEIGHT OF PETROLEUMPITCH CHARACTERIZED BY HAVING A CUBE-IN-WATER SOFTENING POINT BETWEENABOUT 40* AND ABOUT 85*C. AND HAVING A CONTENT OF MATERIAL INSOLUBLE INQUINOLINE NOT GREATER THAN ABOUT 1%.
 2. The process of claim 1 in whichsaid electrolytic cell is an aluminum reduction cell.
 3. The process ofclaim 1 in which said aggregate is calcined anthracite.
 4. The processof claim 1 in which said aggregate is a mixture of calcined anthraciteand calcined metallurgical coke.
 5. The process of claim 1 in which saidaggregate is predominantly finely divided material of which about 50%passes a 200 mesh screen.